There is considerable evidence that competitive interactions play a major role in determining the both the morphological properties of, and distributions of functional synapses made by, afferents to a given central nervous system nucleus. However, it is not at all clear whether this is the case with respect to the trigeminal (V) primary innervation of the rat's brainstem. In fact, limited evidence (Belford and Killackey, '80; Durham and Woolsey, '84) has suggested that such interactions may play little or no role in the postnatal development of this pathway. We will determine whether competitive interactions influence either the pre- and/or postnatal development of V primary afferents by carrying out three experiments. First, we will use anterograde transport of Di-I and horseradish peroxidase (HRP) to label individual V primary afferents in both fetal and neonatal rats. These fibers will be reconstructed in the light microscope and distributions of synapses made by single axons will be determined by electron microscopy. Secondly, we will record from individual V primary afferents and neurons in V nucleus principalis (PrV) in fetal and newborn rats to determine whether the latter cells receive greater convergent input than is the case in adult animals. The results of these two studies will allow us to determine whether the substrate for competitive interactions among V primary afferents exists in fetal or early postnatal life. In a third and final experiment, we will use fetal surgical techniques to damage the peripheral axons of primary afferents that innervate selected vibrissae follicles. If competitive interactions shape the central arbors of vibrissa-related primary afferents, such lesions should put undamaged axons at a "competitive advantage" and increase the size of their central terminal arbors. We will use intra-axonal recording and HRP injection techniques to determine whether or not this occurs.